The invention relates to a diode laser arrangement having a plurality of laser bars, wherein each laser bar has a plurality of emitters generating laser beams arranged offset with respect to one another.
In contrast to conventional laser beam sources which have a beam diameter of a few millimeters with a low beam divergence in the range of a few mrad, the radiation of a semiconductor or diode laser (hereinafter “diode laser”) is characterised by a highly divergent beam in the fast axis having a divergence >1000 mrad. This is caused by the outlet layer limited to <1 μm height, at which a large angle of divergence is produced, similar to the diffraction at a slit-shaped opening. Since the extension of the outlet opening in the plane perpendicular and parallel to the active semiconductor layer is different, different beam divergences come about in the plane perpendicular and parallel to the active layer.
In order to achieve a power of 20-40 W for a diode laser, numerous laser emitters are combined on a so-called laser bar to form a laser component. Usually 10-50 individual emitter groups are arranged here in a row in the plane parallel to the active layer. The resulting beam of such a bar has an aperture angle of about 10° and a beam diameter of about 10 mm in the plane parallel to the active layer. The resulting beam quality in this plane is many times lower than the resulting beam quality in the previously described plane perpendicular to the active layer. Even with a possible future reduction in the angle of divergence of laser chips, the very different ratio of the beam quality perpendicular and parallel to the active layer will persist.
As a result of the previously described beam characteristic, the beam has a very large difference in beam quality in both directions perpendicular and parallel to the active layer. The concept of the beam quality is in this case described by the M2 parameter. M2 is defined by the factor by which the beam divergence of the diode laser beam lies above the beam divergence of a diffraction-limited beam of the same diameter. In the case shown above, a beam diameter which is a factor of 10,000 above the beam diameter in the perpendicular plane is obtained in the plane parallel to the active layer. The situation is different with the beam divergence, i.e. an almost ten times lower beam divergence is achieved in the plane parallel to the active layer or in the slow axis. The M2 parameter in the plane parallel to the active layer is therefore several orders of magnitude above the M2 value in the plane perpendicular to the active layer.
One possible aim of beam shaping is to achieve a beam having almost the same M2 values in both planes, i.e. perpendicular and parallel to the active layer. At the present time, the following methods are known for shaping the beam geometry by which an approximation of the beam qualities in the two principal planes of the beam is achieved.
By means of a fiber bundle, linear beam cross-sections can be combined by rearranging the fibers to form a circular bundle. Such methods are described, for example, in the U.S. Pat. Nos. 5,127,068, 4,763,975, 4,818,062, 5,268,978 and 5,258,989.
In addition, there is the technique of beam rotation in which the radiation of individual emitters is rotated by 90° in order to thus make a rearrangement in which the beams are arranged in the direction of the axis of better beam quality. The following arrangements are known for this method: U.S. Pat. No. 5,168,401, EP 0 484 276, DE 4,438,368. All these methods have in common that after being collimated, the radiation of a diode laser is rotated by 90° in the fast axis direction in order to perform a slow-axis collimation with a common cylindrical optics. As a modification of the said methods, a continuous linear source is also feasible (i.e. that of a diode laser of high surface density, collimated in the fast axis direction), whose beam profile (line) is split and is present in rearranged form after the optical element.
In addition, it is possible to perform a rearrangement of the radiation of individual emitters without any rotation of the beam where a rearrangement of the radiation is achieved, for example, by the parallel offset (displacement) by means of parallel mirrors (WO 95/15510). An arrangement which also uses the rearrangement technique is described in DE 19 5 44 488. In this case, the radiation of a diode laser bar is deflected in different planes and individually collimated there.
The disadvantages of this art can be summarised inter alia in that in fiber-coupled diode lasers a beam having very different beam qualities in both axial directions is usually coupled into the fiber. In the case of a circular fiber this means that in one axial direction the possible numerical aperture or the fiber diameter is not used. This leads to appreciable losses in the power density so that in practice this is restricted to about 104 W/cm2.
In the said known methods, in some cases appreciable path length differences must furthermore be compensated. This is mainly achieved by correction prisms which can only compensate for defects to a limited extent. Multiple reflections furthermore impose increased requirements on alignment accuracy, production tolerances and component stability (WO 95/15510). Reflecting optics (e.g. made of copper) have high absorption values.
Further known is a laser optical system of the genre-forming type for reshaping at least one laser beam bundle using at least two optical reshaping elements disposed successively in the beam path, which are configured as so-called plate fans (DE 10012480 A1).
In the hitherto known diode lasers the radiation power of the diode laser arrangement is limited and specifically in particular by the fact that the available laser bars have a limited length, for example, a length of about 10 mm in their slow axis (plane of the emitter layers) and the typical optical output power of a laser bar, for example, lies in the range of 250 Watts maximum. On account of the dimensions exhibited in particular by the heat sinks used as supports for the laser bars in the direction of the fast axis, in which the laser bars are provided offset with respect to one another in a stack-like manner in the laser diode arrangement and also on account of the need for optical elements which must be provided on the individual laser bars for fast axis collimation, the stack density of the laser bars within the stack comprising these laser bars and the appurtenant supports or heat exchangers is limited.
It is the object of the invention to provide a diode laser which makes it possible to achieve an improved optical power without losses of beam quality.